1. Field of the Invention
The present invention relates to a film position detection apparatus and a film position detection method for detecting shifts of a position of an object in the form of a film such as an image film and an image film projection apparatus for projecting light onto an image film and projecting images recorded thereon.
2. Description of the Related Art
A transport position and other factors of an image film such as a movie film 1 shown in FIG. 1 are detected in a film position detection apparatus for detecting a position and other factors of a film-shaped object continuously or intermittently running.
The movie film (which may be simply referred to as `film` in the following description) 1 shown in FIG. 1 has sprocket holes (perforations) 2 for synchronization of film transport at both ends of the width of the film 1 at regular intervals in sequence. Recording areas 4 and 5 for recording image and audio information are provided in a portion between rows 3A and 3B of the sprocket holes 2.
The film position detection apparatus for detecting a position and other factors of the film 1 as described above is used for a movie film projection apparatus for projecting images recorded on a movie film, for example. The movie film projection apparatuses include an ordinary movie projector and an apparatus for electronization of information on a film (referred to as a telecine apparatus in the following description). FIG. 2 illustrates an example of a telecine apparatus 100 wherein a film position detection apparatus is used.
In the telecine apparatus 100 the film 1 is intermittently transported by a transport system 200 having a plurality of transport sprockets 210 and 220. If the film position detection apparatus detects the recording area 4 of the film 1 stopped at a specific position in a gate section 300, a shutter 420 in a projection section 400 opens so that light is projected from a light source 410. Consequently an image recorded on the film 1 is projected onto an image pickup section 600 including an image pickup device through a projection lens 500.
In the telecine apparatus 100 as described so far, variations are produced frame by frame in positions of the film 1 in the direction of transport and the direction of width when the film 1 is stopped after intermittent transport. The variations result in deflections of projected images and quality thereof is reduced. If a stopping position of the film 1 is precisely detected by the film position detection apparatus, an output produced thereby is used for compensating deflections of images by an optical or electronic means. However, film position detection apparatuses of related arts have problems described below and accuracy is not satisfactory for compensating deflections of images.
The types of film position detection apparatus of related arts will now be described together with problems thereof.
A method shown in FIG. 3 is for detecting a position of transport of the film 1 through detecting an angle of rotation of a sprocket 20a placed near a gate section 300 by an angle of rotation detector 11. However, the method has a problem of reduced position detection accuracy due to changes with time in the shape of the sprocket holes 2 producing play between each teeth of the sprocket 20a and each of the sprocket holes 2 and due to the sprocket 20a placed apart from the gate section 300. Another problem is that the method does not allow detection of a position in the direction of width of the film 1 but detection of a position of transport of the film 1.
An electrostatic capacitance method has been provided for detecting both of a position in the direction of transport and a position in the direction of width of the film 1. Specific examples of the electrostatic capacitance method will now be described with reference to FIG. 4 to FIG. 9. In FIG. 5A and FIG. 5B to FIG. 8A and FIG. 8B, each A and B illustrate cross sections taken along line A-A' and line B-B', respectively, of a detection region 12 in FIG. 4. The detection region 12 is a region corresponding to an edge of the film 1 including the sprocket holes 2. Electrodes for producing electrostatic capacitance (which may be referred to as `capacitance` in the following description) are provided in the region. A position of the film 1 is detected through detecting changes in the capacitance.
A method shown in FIG. 5A and FIG. 5B provides a flat-shaped ground electrode 13 for producing electrostatic capacitance, opposed to detection electrodes 14 to which voltage is applied. The ground electrode 13 is pressed against the film 1 by the action of springs 15 so as to move in the direction of the film thickness. In this method the vertical position of the ground electrode 13 slightly changes when the film 1 advances and stops since the ground electrode 13 contacts the film 1. Therefore the capacitance changes due to variations in the space between the electrodes, which leads to a great error in film position detection.
Reference is now made to FIG. 6A and FIG. 6B illustrating a method wherein a flat-shaped ground electrode 13A is held with an established space from detection electrodes 14A. The space is allowed to be greater than the film thickness so that the film 1 passes smoothly. (For example, the space may be 0.2 mm when the film thickness is 0.16 mm.) A detection error arises in the method since the position of the film 1 is allowed to change in the direction of thickness in the space. The cause of the error is that the dimensions of the detection electrodes 14A are not great enough compared to those of the ground electrode 13A and the space and consequently a proportion of transversely expanding components of electric field E becomes greater between the electrodes as shown in FIG. 9.
A method shown in FIG. 7A and FIG. 7B provides a ground electrode 13B with convexes formed only in parts facing the detection electrodes 14B. The space between the ground electrode 13B and the detection electrodes 14B is narrow. The space is greater between the detection electrodes 14B and the part of the ground electrode 13B where the convexes are not formed so as not to affect generation of capacitance. In this method a proportion of transversely expanding components of electric field E as in the method shown in FIG. 6A and FIG. 6B is reduced. As a result, variations in capacitance due to shifts in the position of the film 1 in the space in the direction of film thickness are reduced. Detection accuracy is thus improved compared to that of the method shown in FIG. 6A and FIG. 6B. However, the accuracy is still not satisfactory. Another problem is that with the convex ground electrode 13B the position detection output changes with variations in the location of the electrode on the film surface (in the directions of film transport and width). Position detection is thus affected.
Still another problem is that it is difficult to achieve repeatability of the electrode position (of the order of microns) to prevent detection errors since the ground electrode 13B is moved off the detection electrodes 14B when the film is placed or removed.
Another method is shown in FIG. 8A and FIG. 8B, providing a frame-shaped film retainer 16 formed around a convex ground electrode 13C for pressing the film 1 against detection electrodes 14C. The film retainer 16 suppresses shifts in the position of the film 1 in the direction of the thickness in a fixed space. In this method accuracy is improved in the position of the film 1 in the direction of the thickness, compared to the method described above providing the convex ground electrode 13B only. However, detection accuracy of the method is still not satisfactory since the film 1 is not pressed directly on the detection electrodes 14C and shifts remain in the position of the film 1 in the direction of the thickness over the electrodes. Another problem is that the film 1 may receive flaws due to friction produced by the film retainer 16 pressing the film 1 with narrow areas. Still another problem is that noise is added to the detection output due to static electricity produced between the film 1 and the film retainer 16.